A Pew Hispanic analysis finds that 35 percent of unauthorized adult immigrants have resided in the U.S. for 15 years or more; 28 percent for 10 to 14 years; 22 percent for 5 to 9 years; and 15 percent for less than five years.

The share that has been in the country at least 15 years has more than doubled since 2000, when about one-in-six or 16 percent unauthorized adult immigrants had lived here for that duration. By the same token, the share of unauthorized adult immigrants who have lived in the country for less than five years has fallen by half during this period—from 32 percent in 2000 to 15 percent in 2010.

The decrease has occurred in part because of reduced flows into the U.S. among Mexicans, who constitute 58...